1. Field of the Invention
The present invention relates to a chemical vapor deposition process for forming bismuth-containing material films on a substrate for applications such as the manufacture of ferroelectric memory devices.
2. Description of the Related Art
In recent years, ferroelectric materials have been the focus of widespread interest as components of non-volatile memory devices.
Non-volatile ferroelectric memory devices function by storage of information through the polarization of a thin ferroelectric material layer reposed between two plates of a capacitor structure. Each such ferroelectric capacitor is connected to a transistor to form a storage cell, in which the transistor controls the access of the read-out electronics to the capacitor. The transistor therefore is connected to bit-line and word-line elements, to constitute the storage cell.
The ferroelectric material may be utilized in a stacked capacitor structure which overlies the top of the transistor. The transistor drain (e.g., of a MOSFET structure) is connected to the bottom electrode of the capacitor by a plug formed of suitable materials such as polysilicon or tungsten.
Information subsequently can be changed in the ferroelectric memory cell by applying an electric field to the thin ferroelectric material layer to reverse (xe2x80x9cflipxe2x80x9d) the polarization characteristic of the ferroelectric material. Ferroelectric memories (FRAMs), in contrast to dynamic random access memories (DRAMs), have the advantages of retaining stored information in the event of termination of the power supply, and do not require refresh cycles.
In such memory applications, ferroelectric materials desirably have the following electrical properties: (1) a low coercive field characteristic, facilitating use of a low voltage power supply; (2) a high remnant polarization characteristic, ensuring highly reliable information storage; (3) absence of significant fatigue or life-time deterioration characteristics; (4) absence of any imprint which would alter the stored information (e.g., leading to a preference of a certain polarization such as a logical xe2x80x9c1xe2x80x9d over a logical xe2x80x9c0xe2x80x9d character) or otherwise impair the ability to xe2x80x9creadxe2x80x9d the stored information; and (5) extended retention time, for reliable data storage over an extended period of time.
The foregoing electrical property criteria are satisfied in the layered pseudo-perovskite or xe2x80x9cAurivilliusxe2x80x9d phase of materials such as strontium bismuth tantalate, SrBi2Ta2O9. This strontium bismuth tantalate composition is sometimes hereinafter referred to as xe2x80x9cSBT.xe2x80x9d As a result of these favorable characteristics, significant efforts have been initiated to integrate SBT in memory devices. Ferroelectric capacitor structures utilizing SBT as the ferroelectric material have been made in the prior art by sol-gel techniques and demonstrate superior electrical properties.
Unfortunately, however, such sol-gel methodology permits only a low integration density to be achieved. Some improvement in the sol-gel methodology may be gained by mist or electrospray methods, permitting fabrication of memories up to 4 megabit in capacity.
To achieve higher integration density of SBT with smaller structure sizes (e.g., having a minimal feature size below about 0.7 micron), it is necessary to utilize chemical vapor deposition (CVD) processes, since CVD affords better conformality and step coverage than layers produced by any other deposition method. Further, the CVD process yields deposited films having a high film uniformity, high film density, and the capability to grow very thin films at high throughput and low cost.
Concerning relevant art to the present invention, U.S. Pat. No. 5,527,567 to Desu et al. discloses CVD of a ferroelectric layered structure in a single-step or a two-step process at temperatures which in the single-step process may be in the range of 450-600xc2x0 C., and in the two-step process may be in the range of 550-700xc2x0 C. in the first step and 600-700xc2x0 C. in the second step. The Desu et al. patent describes post-deposition treatment steps including annealing, deposition of a top electrode, and further annealing.
Desu et al. describe as suitable precursors for the respective bismuth, strontium and tantalum components of the SBT film triphenyl bismuth, strontium bis(2,2,6,6-tetramethylheptane-2,5-dionate)tetraglyme adduct and tantalum pentaethoxide, with such precursors being utilized in a solvent medium such as an 8:2:1 mixture of tetrahydrofuran, isopropanol and tetraglyme. The Desu et al patent mentions Bi(thd)3.
The Desu et al patent describes vaporization at temperatures of 60-300xc2x0 C. In a specific embodiment, Desu et al. teach evaporation of the precursors in a vaporizer by direct liquid injection, at a temperature in the range of 250-320xc2x0 C., followed by growth of films from the vaporized precursor at a temperature in the range of 450-800xc2x0 C. on a Pt/Ti/SiO2/Si or sapphire substrate, with post-deposition annealing at 750xc2x0 C.
A related patent of Desu et al. is U.S. Pat. No. 5,478,610.
U.S. Pat. No. 5,648,114 describes a method of fabricating an electronic device including CVD deposition of a layered superlattice material which is post-treated to improve its properties.
The art continues to seek improvement in SBT and ferroelectric memory technologies.
It is an object of the present invention to provide an improved CVD process for the deposition of bismuth oxide materials (e.g., SrBi2Ta2O9, Bi4Ti3O12, etc.), including materials of such type for formation of ferroelectric thin films for applications such as ferroelectric memory devices.
Other objects of the invention will be more fully apparent from the ensuing disclosure and appended claims.
The present invention relates to an improved chemical vapor deposition process for forming bismuth-containing films on substrates for applications such as ferroelectric memory devices.
In one aspect, the present invention relates to a process for depositing bismuth-containing material on a substrate by chemical vapor deposition from a precursor which is vaporized to form a precursor vapor which is then contacted with the substrate. The precursor for depositing the bismuth-containing material is a bismuth xcex2-diketonate.
In one specific aspect of the invention, the precursor for forming the bismuth-containing material on the substrate comprises an anhydrous mononuclear bismuth xcex2-diketonate, a new material.
In a further specific aspect, the present invention relates to the formation of a bismuth-containing material by chemical vapor deposition using Bi(thd)3.
In another aspect, the invention relates to formation of a bismuth-containing ferroelectric film such as SBT, including deposition of bismuth from a bismuth xcex2-diketonate precursor of the formula BiAxBy wherein A=xcex2-diketonate, B=another ligand which is compatible with the use of the composition as a precursor for formation of bismuth-containing material on a substrate, x=1, 2 or 3, and y=3-x.
In the practice of the present invention, the xcex2-diketonate ligand of the anhydrous mononuclear tris(xcex2-diketonate) bismuth composition may be of any suitable type, including the illustrative xcex2-diketonate ligand species set out in Table I below:
The composition and the synthesis of such bismuth tris(xcex2-diketonate) precursors are more fully disclosed and claimed in co-filed U.S. patent application No. [File: ATM-256] filed Oct. 30, 1997 in the names of Thomas H. Baum, Gautam Bhandari and Margaret Chappuis for xe2x80x9cAnhydrous Mononuclear Tris (xcex2-Diketonate) Bismuth Compositions for Deposition of Bismuth-Containing Films, and Method of Making the Same,xe2x80x9d the disclosure of which hereby is incorporated herein by reference in its entirety.
In such deposition of bismuth on a substrate, for applications such as the formation of SBT ferroelectric thin films, the use of the anhydrous mononuclear bismuth source material provides improved thermal transport and flash vaporization.
Although the mononuclear bismuth source material is highly preferred in the practice of the present invention, the invention also contemplates the use of binuclear bismuth precursors, as well as mixtures of mononuclear and binuclear bismuth source materials.
In another aspect, the present invention relates to a method of forming an SBT film on a substrate by chemical vapor deposition from precursors for the strontium, bismuth and tantalum constituents, wherein the bismuth precursor comprises a bismuth tris (xcex2-diketonate) complex. The strontium and tantalum precursors may be of any suitable precursor types as source materials for such metal components, but strontium (2,2,6,6-tetramethyl-3,5-heptanedionato)2(L), wherein the ligand L is tetraglyme or pentamethyldiethylenetriamine, is a preferred Sr precursor, and the tantalum precursor most preferably comprises Ta(OiPr)4(2,2,6,6-tetramethyl-3,5-heptanedionate).
For liquid delivery, the above-discussed SBT precursors may be mixed with any suitable solvent medium, e.g., an 8:2:1 mixture of tetrahydrofuran/isopropanol/tetraglyme, or, more preferably, a 5:4:1 ratio mixture of octane, decane and pentamethyldiethylenetriamine, with the respective Sr, Bi and Ta precursors being individually stored (in solution) in separate reservoirs or supply vessels.
Prior to delivery, the three precursor solutions may be mixed in a liquid delivery system such as that disclosed in U.S. Pat. No. 5,204,314 issued Apr. 20, 1993 in the names of Peter S. Kirlin, et al., the disclosure of which hereby is incorporated herein by reference in its entirety. The resulting precursor liquid mixture then may be vaporized at suitable temperature in the range of for example 150-240xc2x0 C., optionally with argon or other carrier gas to transport the resulting multicomponent precursor vapor to the CVD reactor for contacting with the hot substrate. The carrier gas may be mixed with or initially contain oxygen or other oxidant gas.
The precursor vapor in the CVD reactor is contacted with the substrate, as for example a Pt/Ti/SiO2/Si wafer at a temperature which for example may be on the order of 300-450xc2x0 C., for sufficient time to permit film growth to occur to the desired extent.
The SBT film formed by such method may be an amorphous film or an oriented fluorite-containing film (fluorite usually being oriented, but not always highly), or a combination of the two forms. The amorphous film when present can be transformed to a fluorite structure which can then be transformed to the desired Aurivillius phase by a post-deposition anneal process.
Alternatively, the amorphous film may be post-treated to convert same directly to the Aurivillius phase. The annealing temperatures usefully employed to transform an SBT film deposited from a bismuth xcex2-diketonate precursor into a ferroelectric Aurivillius phase are in the range of about 600 to about 820xc2x0 C.
The solvent mixture of 8:2:1 tetrahydrofuran/isopropanol/tetraglyme described above is illustrative, and other solvent mixtures may be employed. A particularly preferred solvent mixture for the precursors is a 5:4:1 octane/decane/pentamethyldiethylenetriamine mixture.
While the method of the invention is primarily addressed herein in relation to formation of ferroelectric SBT films and device structures on a substrate, the invention is also generally applicable to the formation of other ferroelectric films, including bismuth-containing strontium-based materials, bismuth-containing titanium-based materials and other bismuth-containing film materials including other metallic and oxide components.
Other aspects and features will be more fully apparent from the ensuing disclosure and appended claims.